Light emitting diodes (LEDs) are widely used in lighting and backlighting systems. Specifically, LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions. In a large LCD backlight system, LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current.
Supplying a white backlight with colored LEDs is achieved by placing one or more individual strings of colored LEDs in proximity so that in combination their light is seen as white light. The white point of the light is an important factor to control, and much effort in design is centered on the need to maintain a correct white point.
FIG. 1 shows a high level schematic block diagram of a backlighting system 10, (not in the drawing) according to the prior art, comprising a color manager 100, a sampler 110 and a luminaire 120. Color manager 100 comprises: a current driver 130; a converter 140 comprising a calibration matrix 150; and a difference module 160. Current driver 130 is coupled to luminaire 120. Luminaire 120 comprises strings of colored LEDs in proximity such that the red, green and blue LEDs are combined to produce a white light or any other predetermined color. Luminaire 120 is in optical communication with sampler 110, typically constituted of a digitizer in communication with an RGB photodiode assembly. The RGB photodiode assembly may be integrated within sampler 110, or provided as a separate sensor in communication with sampler 110. Sampler 110 outputs a tristimulus output, wherein each signal represents the intensity of a sampled color, denoted as RS, GS and BS, which is coupled to converter 140.
Converter 140 outputs a tristimulus output, wherein each signal represents a value in a predetermined color space, denoted XS, YS and ZS. The tristimulus output of converter 140 is coupled to the input of difference module 160. Difference module 160 further receives external signals, denoted XT, YT and ZT, corresponding to the desired output color point and intensity of luminaire 120, and outputs signals responsive to the difference between signals XS, YS, ZS and XT, YT, ZT, respectively, denoted ERROR1, ERROR2 and ERROR3. The external signals XT, YT and ZT may be generated by a video processor or controller (not shown) that is coupled to color manager 100.
Converter 140 is operative to convert the sampled light output of sampler 110 in cooperation with calibration matrix 150. Calibration matrix 150 is typically determined by a single pass of calibration values during manufacture, in which each of the colored LED strings of luminaire 120 are lit independently at a predetermined duty cycle, the output of sampler 110 is read, and the output of luminaire 120 is measured by a calorimeter arranged to express the measured output values in a desired calorimetric system. There is no requirement that CIE values be used, and any consistent calorimetric system may be utilized. The values measured by the calorimeter are compared with the output of sampler 110 and calibration matrix 150 is developed responsive thereto.
Since RS, GS, BS and XT, YT, ZT are not defined over the same color space, color manager 100 is arranged to convert sampled color intensity values RS, GS and BS into a calorimetric system consonant with calorimetric system of the target color point using converter 140. This is accomplished in cooperation with calibration matrix 150, which is defined at a particular operating point of luminaire 120, typically during the calibration process.
Color manager 100 is thus arranged to maintain at any given time the color point of operation of luminaire 120 responsive to the received external signals. Maintaining a predetermined color point is achieved by: sampling the combined output of the colored LED strings using sampler 110; converting the sampled output to a calorimetric system consonant with the received external signals using converter 140; comparing the converted sampled output with the received external signals using difference module 160; and correcting the output color point of luminaire 120, if required, by controlling the driving current of current driver 130.
As mentioned above, color manager 100 is fed with externally supplied signals XT, YT and ZT which define the target color point output of luminaire 120, and typically further define the intensity. It is to be understood that a backlight may be comprised of a plurality of zones, each comprising therein a luminaire 120 and sampler 110, with an associated color manager 100. A single color manager 100 may be associated with a plurality of zones.
The intensity of the colored LED strings constituting luminaire 120 may be controlled by amplitude modulation (AM) of the driving current and/or by pulse width modulation (PWM). Specifically, in AM the value of the constant current driving the LEDs is adjusted, while in PWM, the duty cycle is controlled to dim or brighten the LEDs of the LED string by adjusting the average current over time. Thus, color manager 100 may adjust either AM or the PWM of each colored LED string of luminaire 120 responsive to ERROR1, ERROR2 and ERROR3, consequently maintaining the desired color point.
Additionally, a user or controlled dimming input may be received. Dimming may be accomplished by further modifying the AM or PWM of each colored LED string of luminaire 120 without modifying the desired color point.
The use of AM unfortunately results in changes to the spectral output of constituent LEDs of luminaire 120, since the spectral output of the constituent LEDs is sensitive to the driving current level. Furthermore, there is a tendency of colored LEDs to change their output as a function of temperature, age and other environmental conditions. One method of overcoming this difficulty is by exclusively using PWM at a fixed current value. Backlighting system 10 advantageously controls the color point responsive to a calibration matrix established at a predetermined driving point. The use of AM modulation in backlighting system 10 results in an improper color point, since calibration matrix 150 is defined at a predetermined current value.
LCD based monitors and televisions are placed in various settings, in which ambient light may change. Dimming of the backlight responsive to the ambient light setting is known to the prior art. However the exclusive use of PWM dimming, as described above, limits the range of brightness settings available to the user in cooperation with ambient light based dimming, since very low PWM settings may result in flicker, and maximum brightness achieved at high PWM settings is typically less than that available by increasing the instantaneous current.
Certain LCD based monitors exhibit a scanning backlight mode, in addition to a non-scanning backlight mode. In the scanning backlight mode the backlight is turned on and off at fixed intervals, resulting in a reduced overall luminance. It would thus be desirable to adjust the brightness responsive to the selection of scanning and non-scanning backlight mode. However the exclusive use of PWM dimming, as described above, limits the range of brightness settings available to the user in cooperation with ambient light based dimming.
It would thus be advantageous to have a color manager that is capable of operation both with PWM dimming and AM.